Onan Generator Oil: Which OnaMax Grade Does Your RV Need?
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Onan, the generator brand owned by Cummins, powers a huge share of the built-in generators found in motorhomes and travel trailers. Onan sells its own engine oil under the OnaMax name, and many owners stick with it because the owner’s manual names it and because it matches the oil used in Onan’s maintenance kits. If you just bought your first RV, the generator oil change is one of the easiest jobs to learn, and the first step is buying the right grade.
The six products below include four genuine Onan and Cummins oils in two grades, 15W-40 and 10W-30, plus two third-party service parts, an oil pressure switch and a tune-up kit, that are not oil at all but often end up in the same shopping cart.
Onan Generator Oils and Service Parts Compared
| Product | Type | Grade or part number | Amount | Maker |
|---|---|---|---|---|
| Onan OnaMax SAE 15W-40 Engine Oil | Engine oil, conventional | 15W-40, part 326-5336 | 32 oz | Cummins |
| Cummins 3265336 Onan SAE 15W-40 Oil | Engine oil | 15W-40, part 3265336 | 3 x 1 quart | Cummins |
| Cummins Onan OnaMax 10W-30 Generator Engine | Engine oil | 10W-30, part A063E183 | 1 quart | Cummins |
| Onan Cummins 326-5336 OEM RV OnaMax | Engine oil | 15W-40, part 326-5336 | 1 quart | Onan |
| Oil Pressure Switch 187-6265 PS131 0965 | Replacement switch | Replaces 187-6265 | 1 switch | FEFAXIQI (third-party) |
| GPS GENERATOR PARTS SPECIALIST Tune Up | Tune-up kit with oil | For KY 4000, Spec A-N | Not listed | GPS Generator Parts Specialist (third-party) |
Onan OnaMax SAE 15W-40 Engine Oil for Quick Top-Offs
Cummins
Onan OnaMax SAE 15W-40 Engine Oil, 32 oz.
Price and specifications are read from the Amazon product page and can change.
Onan OnaMax SAE 15W-40 Engine Oil
Best for: Topping off a 15W-40 generator
Pros
- Genuine OnaMax 15W-40
- Formulated to reduce oil consumption
- Single 32-oz bottle
Cons
- Conventional, not synthetic
- One bottle may not cover full change
The Onan OnaMax SAE 15W-40 Engine Oil is the familiar 32-ounce bottle, which is the same as one quart, carrying Cummins part number 326-5336. The listing describes it as a conventional, non-synthetic oil, specially formulated to reduce oil consumption and engine wear caused by oil breaking down at high temperatures.
The grade name tells you how the oil flows. The first number with the “W” (15W) describes how it behaves when cold, with lower numbers flowing more easily at startup. The second number (40) describes how thick it stays once the engine is hot. A 15W-40 oil stays relatively thick in heat, which is why it is often chosen for warm-weather running.
This single bottle suits an owner who needs to top off between changes or keep a spare in the RV’s storage bay. Many RV generators need more than a quart for a full change, so check your manual for capacity; if it is more than one quart, the three-pack below saves a second order.
Cummins 3265336 Onan SAE 15W-40 Oil for Full Oil Changes
Cummins
Cummins 3265336 Onan SAE 15W-40 Oil – 1 Quart (Quantity 3)
Price and specifications are read from the Amazon product page and can change.
Cummins 3265336 Onan SAE 15W-40 Oil
Best for: Full oil changes with spare quarts
Pros
- Three 1-quart bottles
- Same 326-5336 part family
- 15W-40 viscosity grade
Cons
- Listing gives almost no detail
- Only for 15W-40 generators
The Cummins 3265336 Onan SAE 15W-40 Oil comes as three 1-quart bottles of the same grade and part family as the single bottle above. The listing is thin on detail: beyond the title, it gives the 15W-40 viscosity grade, a medium viscosity level and compatibility with trucks, buses and generators.
The liquid volume line reads 32 fluid ounces, which appears to describe each bottle rather than the whole pack, but the listing does not say that directly. What you can count on from the title is quantity three.
This suits an owner doing a full oil change at home or someone who wants a season’s supply in one order. It also makes a practical gift for an RV owner who already knows their generator takes 15W-40. If their manual calls for 10W-30, skip to the next pick.
Cummins Onan OnaMax 10W-30 Generator Engine Oil for Cooler Starts
Cummins
Cummins Onan OnaMax 10W-30 Generator Engine Oil 1Qt A063E183
Price and specifications are read from the Amazon product page and can change.
Cummins Onan OnaMax 10W-30 Generator Engine
Best for: Generators specifying 10W-30, cooler climates
Pros
- Genuine part A063E183
- For gasoline and LP generators
- Used in Onan maintenance kits
Cons
- Single quart only
- Listed weight looks inconsistent
The Cummins Onan OnaMax 10W-30 Generator Engine oil is the other OnaMax grade, sold in a 1-quart bottle with genuine part number A063E183. The description says it is for RV gasoline and LP generator engines, and the listing notes it is used in several current Cummins Onan maintenance kits.
The lower first number, 10W, means the oil flows more easily when the engine is cold, which is why 10W-30 is commonly chosen for cooler weather and cold starts. The listing itself asks you to confirm the required viscosity in your generator’s operator manual before use, and that is the rule to follow. One oddity: the listing gives an item weight of 24.1 pounds for a single quart, which cannot be right, so ignore that line.
This suits owners whose manual specifies 10W-30 or who camp in cool climates where the manual allows it. For hot-weather running where 15W-40 is specified, the other OnaMax bottles are the match.
Onan Cummins 326-5336 OEM RV OnaMax as the RV-Labeled Quart
Onan
Onan Cummins 326-5336 OEM RV OnaMax 1 Quart Bottle Engine Oil – Fuel Save Consumption Reduction – SAE 15W-40z
Price and specifications are read from the Amazon product page and can change.
Onan Cummins 326-5336 OEM RV OnaMax
Best for: RV owners wanting the OEM quart
Pros
- OEM part 326-5336
- Listed for RV use
- 15W-40 viscosity grade
Cons
- No feature details in listing
- Single quart only
The Onan Cummins 326-5336 OEM RV OnaMax quart carries the same 326-5336 part number as the first pick, sold under the Onan brand name. OEM stands for original equipment manufacturer, meaning it is the oil the generator maker itself supplies. The title mentions fuel saving and consumption reduction, and the specifications list SAE 15W-40, a 1-quart bottle and compatibility with RVs.
The listing has no feature bullets, so everything we know comes from the title and specification table. It is the same grade you would get from the Cummins-branded bottle, just listed with RV wording.
This suits someone searching specifically for the Onan-branded RV quart. If you need several quarts at once, the three-pack is the more convenient order.
Oil Pressure Switch 187-6265 PS131 0965 for Low-Oil Shutdown Repairs
Best for: QG owners replacing a failed switch
Pros
- Replaces part 187-6265
- Metal body
- Plug-in connector
Cons
- Third-party, not Onan-made
- Listing has confusing fitment text
The Oil Pressure Switch 187-6265 PS131 0965 is not oil and is not made by Onan. It is a third-party replacement from FEFAXIQI for the oil pressure switch on Onan QG models HGJAA through HGJAF and on EH650V gas engines. An oil pressure switch tells the generator when oil pressure drops, so the engine can shut down before damage occurs. A failing switch can cause a generator to shut off even when the oil level is fine.
The listing names a metal body, a plug-in connector and a normally closed contact type, and says it replaces part numbers 1876265, PS131, 491657S and MC840219. Be aware the listing is messy: one bullet mentions a Nissan Rogue, and the temperature ratings read 1°C for both low and high, which is clearly a data error. The seller asks buyers to compare part numbers before ordering and offers replacement or refund if it fails.
This suits a QG owner who has already diagnosed a bad switch and matched the part number. It is not something to buy just in case alongside an oil change.
GPS GENERATOR PARTS SPECIALIST Tune Up Kit for KY 4000 Generators
Best for: KY 4000 owners doing a tune-up
Pros
- Bundles OnaMax 15W-40 oil
- Made for one generator model
Cons
- Third-party kit, not Onan
- Contents not detailed beyond title
The GPS GENERATOR PARTS SPECIALIST Tune Up kit is a third-party bundle for the Onan RV generator model KY 4000, Spec A through N, that includes OnaMax 15W-40 oil according to its title. Shoppers often search for this as the Onan KV 4000 tune-up kit, but the listing title spells the model KY 4000, so check your generator’s nameplate carefully.
Only the listing title is available, so the listing does not specify which filters, spark plugs or other parts are in the kit, or how much oil is included. Look at the product page for the contents before ordering.
This suits a KY 4000 owner who wants everything for a service in one box and already knows their generator’s spec letter. For owners of other models, buy the oil on its own.
How to Choose the Right Onan Generator Oil
Start with the manual. Onan specifies the grade for each generator, and the 10W-30 listing itself tells you to verify viscosity before use. Your manual is the final word on which oil to use in an Onan generator.
Match the grade to your climate. As a general rule, 15W-40 stays thicker in heat and suits warm-weather camping, while 10W-30 flows more easily on cold starts. Many owners switch grades by season when their manual allows it.
Buy enough for a full change. An RV generator oil change usually means draining the old oil, replacing the filter and refilling to the dipstick mark. Check your manual for capacity, then buy single quarts or the three-pack to cover it. If you are also exploring backup power at home, our solar generator guide explains a maintenance-free alternative.
Keep service parts separate. The oil pressure switch and tune-up kit are third-party parts for specific models. Buy them only when the part number or model on your generator matches. For a portable backup to your built-in unit, see our 3600-watt inverter generator review.
FAQ
Which oil does an Onan generator use?
Onan’s own OnaMax oil comes in 15W-40 and 10W-30. Your generator’s operator manual says which grade to use.
Is OnaMax 15W-40 synthetic?
No. The 32-oz listing describes it as a conventional, non-synthetic oil.
What is the difference between 15W-40 and 10W-30?
10W-30 flows more easily when cold, which helps cold starts. 15W-40 stays thicker when hot, which suits warm temperatures.
Is a 32-oz bottle the same as one quart?
Yes. A U.S. quart is 32 fluid ounces, so the 32-oz OnaMax bottle and the 1-quart bottles hold the same amount.
Are the oil pressure switch and tune-up kit made by Onan?
No. The switch is from FEFAXIQI and the kit is from GPS Generator Parts Specialist. Both are third-party parts sold for specific Onan models.
How do I know if the tune-up kit fits my generator?
Check the model and spec letter on your generator’s nameplate. The kit is listed for the KY 4000, Spec A through N.
